Data Studio - Custom Dimension

If you've added a custom field previously you should be able to add another without any issues. Are you the owner of the data source? What is the source of your data? Is there any way your permissions could have changed between the time you made the first customer metric and now?

Data Studio - Custom Dimension

That's what I thought first, to be able to make it another custom field :/

Yes I am the owner of the data source, wich is Google Analytics. It is possible that something change indeed but I cannot confirm.

For instance, I have it in french, I was able to customize statistics, while clicking "Add Statistic", then columns or customized data" and then I was given the possibilities to made the ratio Cost/Revenue